First of all, you're right not to want to join SAG right away (read the union's job figures).
SAG vouchers are a bit of a roulette game. As I understand it, it's pretty much at the discretion of production - which can come down to a very distracted assistant in some cases. There's no doubt some route these to people they know or friends of friends; probably people they find attractive from time to time as well. Otherwise, I doubt there's any formula.
It's good to do independent films precisely because you're NOT SAG and can still do the non-unions. You want to do as many of these as you can and then try to beg and scrape copies of your scenes (which is often what it takes). I wouldn't worry too much about whether they'll ever be shown - I doubt the producers know that in many cases. What you want is experience and as much material as you can get for a reel. Not to mention the contacts you can build up with people who themselves might be starting their careers (I once heard - haven't verified - that Robert DeNiro and Harvey Keitel did a student film with Scorcese and then stuck with him from there.)
I wouldn't count on these for SAG credits, though. Either they're already SAG (in which case you might get booked, but it's less likely) or they're either non-union or done under a SAG contract which specifically specifies that non-union does NOT get become SAG by doing the film.
But you should certainly do them where you can (and yes, many are chaotic). It's experience, footage and contacts. All of which will probably be much harder to get once you're bound by union restrictions (unless you're one of the lucky few who starts working soon after you join.)
You could also join AFTRA and then do your best to do ANY principal job under that union. After a year, that would make you SAG-eligible. But you're also not supposed to do non-union work once you join AFTRA, so think about that as well.